1. Field of the Invention
The present invention relates to a resin coated bearing and, more particularly, to a resin coated bearing provided with a resin coat piece for preventing the occurrence of creep, the resin coat piece being formed in a groove extended along the outer circumferential surface of an outer ring of the bearing.
2. Description of the Prior Art
In case where a steel bearing is incorporated in a housing which has a great thermal expansion coefficient such as formed of an aluminum alloy, when the ambient temperature rises, an interference between fitting portions of an outer ring of the bearing and of the housing decreases due to a difference in the thermal expansion coefficients of the both. This results in the occurrence of so-called creep wherein the outer ring of the bearing turns relative to the housing.
For the purpose of preventing the occurrence of such creep, there have been offered resin coated bearings provided with a resin coat piece along the outer circumferential surface of the outer ring thereof (see, for example, Japanese Unexamined Utility Model Publication No.56(1981)-131024 and Japanese Examined Patent Publication No.6(1994)-54131).
FIG. 8 is a sectional view of a principal portion of an example of the prior-art resin coated bearing. Referring to the figure, a resin coated bearing 90 rotatably carries a shaft 94 relative to a housing 93. The resin coated bearing 90 has an annular resin coat piece 92 disposed in a circumferential groove 91a extended along the outer circumferential surface 91b of an outer ring 91 thereof.
As shown in FIG. 9, the resin coat piece 92 is formed by injecting a resin material through a gate 95b into an injection mold 95 loaded with the outer ring 91, the injection mold 95 being formed with a concave groove 95a so configured as to correspond to the circumferential groove 91a of the outer ring 91.
In the resin coated bearing 90, the whole length of the outer circumference of the resin coat piece 92 is projected some tens of micrometers from the outer circumferential surface 91b of the outer ring 91 so that the frictional resistance of the resin coat piece 92 may prevent the outer ring 91 from turning relative to the housing 93 even when the interference between the fitting portions of the housing 93 and the outer ring 91 is decreased.
Examples of the resin material for forming the resin coat piece 92 include polyamide 66 (PA66), polyamide 11 (PA11), polybutylene terephthalate (PBT) and the like.
In order to assure the fitting precision with respect to the housing 93, the resin coated bearing 90 must include the resin coat piece 92 having a high precision of the outside diameter thereof. Unfortunately, the resin coat piece 92 cannot rely only upon the injection molding for assuring the high precision of the outside diameter thereof. Therefore, after completion of the injection molding, the resin coat piece 92 must be subject to a process for grinding the outer circumference thereof for achieving the high precision of the outside diameter thereof. Consequently, the production cost is increased by the cost for the grinding process.
The aforementioned resin materials (PA66, PA11, PBT) used for the conventional resin coat piece 92 have properties as shown in the following Table 1. Based on these properties, each of the resin materials is evaluated as shown in Table 2. Incidentally, the circle in the Table 2 denotes "excellent", whereas the triangle denotes "relatively inferior".
As appreciated from Table 2, however, the resin materials have respective problems although they fully serve the purpose of preventing the creep. More specifically, PA66 has a great dimensional change due to water absorption. PA11 is incapable of withstanding continuous use for a long term at a temperature above 150.degree. C., for example. Furthermore, PA11 is more expensive and less versatile in general as compared with PBT and PA66. PBT, in turn, is incapable of withstanding continuous use for a long term at a temperature above 150.degree. C., for example.
TABLE 1 ______________________________________ Resin types Properties PA66 PA11 PBT ______________________________________ Specific gravity 1.13 1.04 1.31 Water absorption [%] 8.5 2.5 0.4 Tensile strength [Mpa] 83 56 52 Flexural strength 118 66 93 [Mpa] Fusion point [.degree. C.] 260 187 225 Thermal deformation temperature 230 154 154 [.degree. C.] At load of 0.46 Mpa Thermal expansion 8.1 15 10 coefficient [10.sup.-5 /.degree. C.] Cost ratio 1 3.2 1.3 (based on PA66 as 1) ______________________________________
TABLE 2 ______________________________________ Evaluation Resin types item PA66 PA11 PBT ______________________________________ Creep torque .smallcircle. .smallcircle. .smallcircle. Heat .smallcircle. .DELTA. .DELTA. resistance Dimensional .DELTA. .smallcircle. .smallcircle. stability Cost .smallcircle. .DELTA. .smallcircle. ______________________________________